There are many different types of methods for coloring anodic oxide films. For example, coloring by the absorption of pigments such as anilines, coloring by the formation of pigments inside the anodic film by chemical reaction, coloring by electrolytic deposition of metallic compounds by the passage of an electric current, or coloring by optical methods.
The coloring of aluminum by the formation of a porous anodic oxide film and the electrolytic deposition of inorganic particles within the pores of said film has been known for many years. First, anodization of aluminum, one of its alloys, or other light metal, produces a porous metal oxide film (porous anodic layer) on the metal under alternating or direct current flow in an electrolytic bath in which the metal is suspended. Many different organic and inorganic acids may be combined to create a great variety of electrolytes. Typically, sulfuric acid is most commonly used due to its availability, low cost, and low dissolving power.
In a subsequent electrocoloring step, an inorganic material, typically a metal, is deposited in the pores of the metal oxide film by the passage of an electric current. Typically, alternating current is passed between the anodized aluminum substrate and a counter-electrode. The counter-electrode typically consists of graphite or stainless steel; although nickel, copper, and tin electrodes can also be used. The deposition of the inorganic material functions to give the anodized aluminum a colored appearance. In a porous aluminum oxide film, the pores are evenly spaced apart, and there is a barrier layer of aluminum oxide between the bottom of the pores and the surface of the metal. Inorganic metallic pigments deposited in the pores of the aluminum oxide film result in light being scattered from the lower ends of the individual pigment deposits and from the aluminum/aluminum oxide interface. The color produced depends upon the difference in optical path length resulting from separation of the two light scattering surfaces. The pore diameter and barrier layer thickness are directly related to the applied anodizing voltage.
An increase in the size of the deposits and changes in the colors produced can be achieved by modification of the pores adjacent to the barrier layer, In order to obtain coloring by optical interference effects, however, it has been considered necessary, according to the teachings of the prior art, to provide an anodized aluminum in which the thickness of the anodic oxide layer generated during the process has an average size of greater than about 260 .ANG., and a separation distance from the aluminum/aluminum oxide interface, i.e., a barrier layer thickness, of between about 300 .ANG. and about 700 .ANG..
Although the methods for electrolytic coloring of anodized aluminum permit various colors to be obtained, the repertoire of colors produced is often limited to bronzes, blacks and reds. Furthermore, it is often necessary to have a separate coloring bath for each color. In addition, most conventional anodizing procedures use a double anodizing method, exemplified by methods utilizing both sulfuric acid and phosphoric acid-based anodizing solutions to modify the pores of the anodic layer and increase the diameter thereof in order to obtain a larger variety of colors. Use of a second acidic bath solution, such as phosphoric acid, is disadvantageous because it increases the likelihood of contamination by phosphate ions in the electrocoloring method. Contamination by phosphoric acid in this manner may prevent the effective sealing of the final product and lead to the gradual loss of color through weathering.
A method for electrocoloring aluminum with the aim of providing a wider range of colors that was possible with the previous methods is disclosed by Asada, in U.S. Pat. No. 4,022,671, patented on May 10, 1977. The Asada method uses conventional anodizing in sulfuric acid-containing electrolytic bath, followed by a second anodization in a phosphoric acid-containing electrolytic bath under direct current, and coloring in an electrocoloring bath with metallic salts under alternating current. This two-stage anodizing procedure, according to Asada, produces a film that may acquire colors in the range of gray through bronze to black, depending on the treatment time in the electrocoloring bath. Although not expressly recognized by Asada, the treatment of the anodized aluminum by electrolysis in a phosphoric acid bath effects a pore enlargement in the anodic layer, which is responsible for the improvements accomplished by Asada. The method, however, has the above mentioned disadvantage of presenting the serious danger of contaminating the anodized aluminum if suitable means for preventing such contamination are not used, and still leaves much to desire as to the alleged wider range of colors obtained, since said range, although wider than that obtainable with previous methods, is still extremely limited.
Mutsuo Hasegawa et al, in U.S. Pat. No. 4,042,468, patented on Aug. 16, 1977, describes and claims a method for electrocoloring anodized aluminum that mainly comprises treating a previously anodized aluminum piece by electrolysis in a bath containing at least two metallic salts, firstly with direct current and thereafter with alternating current. By the foregoing two steps of electrolysis, Hasegawa claims that the uniformity of the color obtained is much better than that obtained by the prior apt methods, particularly when used for electrocoloring aluminum pieces having protuberant portions that ape normally tinted more intensely than the remainder of the piece. This method, however, is not intended to obtain a wide range of colors in the same bath and, therefore, may only be regarded as a method for improving the uniformity of the colors produced by electrocoloring anodized aluminum, but without any intention of broadening the range of colors obtainable.
The electrolytic coloring of anodized aluminum by means of optical interference effects is disclosed by Sheasby et al in U.S. Pat. No. 4,066,816, patented Jan. 3, 1978 and U.S. Pat. No. 4,152,222, patented May 1, 1979 as a divisional of U.S. Pat. No. 4,066,816. Sheasby teaches a method of coloring anodized aluminum that might be considered as being of the same type of the above described method of Asada. However, the range of colors obtainable by Sheasby is much better than that taught by Asada. This is accomplished by Sheasby by means of the incorporation of a pore enlargement step in the electrolytic process, with the purpose of increasing the diameter of the pores of the anodic layer, particularly at the bases thereof, so as to obtain diameters of over 260 .ANG.. The pore enlargement step of Sheasby may be effected by dissolving the surfaces of the pores of the anodic layer by either chemical or electrochemical means, the latter being preferred to produce a field-assisted dissolution at the base of the pores without much bulk film dissolution, and/or by the growing of an additional anodic layer at the base of the existing anodic layer by the use of an increased voltage of direct current, which increases the length of the enlarged base portions of the pores, as well as the distance between the aluminum/aluminum oxide interface and the bottom of the pores of the existing anodic layer, namely, the so called barrier layer. The pore enlargement step of Sheasby is carried out by treating the previously anodized aluminum by electrolysis in a bath containing an acid having a high dissolving power for aluminum oxide and, depending on the voltage used, the pores of the anodic layer are enlarged, particularly by widening the diameter thereof at their bases, and/or by the growing of a new anodic film under the existing anodic film in order to increase the length of the base portions of the pores and the thickness of the barrier layer. For this step, Sheasby normally uses phosphoric acid, although other acids having a high dissolving power for aluminum oxide can be used. Sheasby finally treats the thus obtained product by electrocoloring the same using metal salts to deposit metallic deposits on the bottom of the enlarged pores of the anodic layer. In order to produce the desired optical interference effect and, therefore, to obtain a wider range of colors, Sheasby requires the presence of large shallow inorganic pigment deposits, preferably accompanied by an increased distance between said deposits and the aluminum/aluminum oxide interface. The deposits must have outer ends of an average size in excess of 260 .ANG. and must be at a distance of 500-3000 .ANG. from the aluminum/aluminum oxide interface. Therefore, Sheasby requires to enlarge the pores of the anodic layer through the use of an electrolytic bath having an acid, such as phosphoric acid, that has a high dissolving power for the aluminum oxide. The use of such acids, however, requires a strict control of the procedure and the acid must be promptly neutralized in order to avoid undue enlargement of the pores and rendering the anodic layer crumbly. On the other hand, use of phosphoric acid tends to contaminate the product and increases the likelihood of unduly redissolving the previously created barrier layer in subsequent process steps, primarily due to the acids remaining in the pores after pigment deposition.
Sheasby et al, in U.S. Pat. No. 4,251,330, patented Feb. 7, 1981, further disclose a process for electrocoloring anodized aluminum which is capable of producing brighter and more intense colors than the process of U.S. Pat. 4,152,222 discussed above. For this purpose, Sheasby carries out the pore enlargement step by the use of an electrolytic bath containing phosphoric acid or other acid having similar strong dissolving power for the aluminum oxide and by the use of alternating current. Although according to Sheasby the whole treatment in the phosphoric acid-based electrolyte is performed under A.C. conditions, in some circumstances a short preliminary D.C. treatment can be employed with advantage. Also according to Sheasby, the electrolyte used for the pore enlargement step may also contain salts of one or more metals, such as tin, nickel or copper, from which coloring deposits are formed during the course of the A.C. treatment. When the electrolyte is free of metal salts which can form pigmentary deposits under A.C., a short anodic D.C. treatment can be employed after the A.C. treatment to slightly thicken the barrier layer in order to reduce the current density in the subsequent coloring stage. This D.C. treatment, however, according to Sheasby, should only be continued for a short time because it results in progressive loss of the advantages of the A.C. treatment. When metal salts are used for coloring purposes in the electrolyte, the D.C. treatment must not be used because it causes redissolution of the coloring deposits. The process described by Sheasby in U.S. Pat. No. 4,251,330, although accomplishing the obtention of brighter and more intense colors as compared with the process of U.S. Pat. No. 4,066,816 and its divisional 4,152,222, still relies on the principles of pore enlargement to provide shallow and extended pigmentary deposits and of barrier layer thickening with the purpose of spacing apart the diffracting surface of the deposits and the aluminum/aluminum oxide interface. This is again accomplished by the use of phosphoric acid or other acid having similar dissolving power for the aluminum oxide and, therefore, it shows the same drawbacks already discussed above.
Sheasby et al, in U.S. Pat. No. 4,310,586, patented Jan. 12, 1982, describes a process for coloring anodized aluminum which is based on the same principles described above, the difference being that, in this particular instance, Sheasby stresses the fact that, in order to increase the distance between the pigmentary deposits and the aluminum/aluminum oxide interface, a second anodic layer of aluminum oxide is formed under the primary anodic layer, that is, under the original barrier layer formed between the bottoms of the pores of the primary anodic layer and the aluminum/aluminum oxide interface. Also, in order to reduce the likelihood of redissolution of the pigmentary deposits when the said second anodic layer is deposited, aided by the residues of the acid used for the pore enlargement step, Sheasby prefers to use metal salts that will form an acid resistant alloy and also prefers to immediately dip the work in a fixative bath, such as a chromate bath, although Sheasby expressly considers this dipping as inconvenient in a commercial operation. The additional anodic layer may be formed as a porous layer or also as a non-porous film indistinctly, which points to the fact that Sheasby is adding this second anodic layer with the sole purpose of increasing the distance between the relatively shallow pigmentary deposits within the enlarged pores of the primary anodic layer, and the aluminum/aluminum oxide interface. The process described in this patent, therefore, also heavily relies on the principle of pore enlargement to increase the area of the pigmentary deposits and to decrease their height, and on the principle of increasing the distance between said shallow deposits and the aluminum/aluminum oxide interface. The use of phosphoric acid of other acid having a similar high dissolving power for aluminum oxide, however, causes exactly the same drawbacks already discussed above, and the attempts of Sheasby to overcome the possibility of redissolution of the pigmentary deposits and of controlling said deposits in order to obtain uniform and stable colors, by dipping the work in a fixative bath and by using acid-resistant metals for providing said pigmentary deposits, cannot be regarded as satisfactory from the commercial point of view, as expressly admitted by Sheasby. On the other hand, none of the processes described by Sheasby et al is able to produce a full color range within the visible spectrum, and the stability of the colors obtained can be achieved only through costly additional fixation steps or through the use of relatively limited ranges of metal salts that can produce acid-resistant alloys upon their electrolytic deposition at the bottom of the enlarged pores. The disadvantages of all the processes described by Sheasby, however, can be considered as caused by the belief of the prior art that control of brightness, intensity and range of colors could only be accomplished by providing relatively shallow, wide pigmentary metal deposits, for which purpose a pore enlargement step was considered as mandatory, thus requiring the use of an electrolyte containing an acid having a strong dissolving power towards aluminum oxide, such as phosphoric acid and the like.
Other electrolytic procedures use complex wave forms, such as asymmetric sine waves, to increase the quality of the final product by producing more consistent colors. These wave forms, however, are generally complex and require expensive equipment making their use impractical.
None of the processes of the prior art, however, is capable of producing a full range of colors within the visible spectrum without the change of coloring baths, and furthermore, none of said prior art processes is capable of producing, in a practical manner, colored aluminum products having stable and uniform colors, unless relatively difficult fixation steps or acid-resistant coloring metals are used, with the consequent increase in the costs and decrease in the efficiency of production.